Field
The present invention relates to wireless communication, and more particularly, to a method and an apparatus of in-sequence delivery considering a multi-flow in a wireless communication system supporting dual connectivity.
Discussion of the Background
Particularly many communication demands are generated in a specific area such as a hotspot in a cell and receiving sensitivity of radio waves may deteriorate in a specific area such as a cell edge or a coverage hole. With the development of wireless communication technology, small cells, for example, a pico cell, a femto cell, a micro cell, a remote radio head (RRH), a relay, a repeater, and the like are together installed in a macro cell for the purpose of enabling communication in areas such as the hot spot, the cell edge, and the coverage hole. Such a network is called a heterogeneous network (HetNet). In a heterogeneous network environment, relatively, the macro cell is a cell having large coverage and the small cells such as the femto cell and the pico cell are cells having small coverage. Since the small cells such as the femto cell and the pico cell use lower power than the macro cell, the network is also referred to as a low power network (LPN). In the heterogeneous network environment, coverages of multiple macro cells and small cells overlap with each other.
User Equipment (UE) can perform wireless communication through two or more base stations among base stations constituting at least one serving cell. This is referred to as dual connectivity and a wireless communication system supporting the dual connectivity is referred to as a dual connectivity system. In this case, one of a plurality of base stations physically or logically divided, which constitute the dual connectivity may be a macro base station (alternatively, an anchorage station) and the other one may be a small base station (alternatively, an assisting base station). The UE may be ‘dually connectable UE’ in which dual connectivity is possible and ‘dually inconnectable UE’ in which dual connection supporting is impossible.
The macro base station manages data flow control, security, and header compression according to a packet data convergence protocol (PDCP) for data transmitted to the small base station through a radio bearer (RB).
In general, the wireless communication system is a single flow structure in which a service is provided to the UE through one RB for one EPS bearer service. However, in the case of the wireless communication system supporting the dual connectivity, the service of one EPS bearer can be provided to the UE through not one RB but two RBs established in the macro cell and the small cell, respectively. That is, the service can be provided to the UE through multi-flows.
In the case of an acknowledged mode (RLC AM), an RLC entity of the UE reorders RLC packet data units (PDUs) when received RLC PDUs are received out of the order in a downlink. In the case of the RLC AM, a transmitting side can retransmit an RLC PDU of which receiving is missed at a receiving side again. The RLC entity reassembles an RLC service data unit (SDU) based on the reordered RLC PDU and sequentially the reassembled RLC SDU to an upper layer (that is, PDCP entity). In the case of the RLC AM, the RLC PDUs can be sequentially delivered through reordering and retransmission methods of the RLC PDUs. In other words, the PDCP entity needs to be sequentially delivered with the RLC SDUs except for reestablishment of the lower layer. However, in the case of the UE in which the multi-flows are constituted, the RLC entity for the small base station and the RLC entity for the macro base station are distinguished to receive the respective RLC SDUs and acquire the PDCP SDU and deliver the acquired PDCP SDU to the upper layer and in this case, the PDCP entity cannot sequentially acquire the PDCP SDUs. Accordingly, in the case of the UE using the multi-flows, a new method for sequential delivery of the PDCP SDUs to the upper layer in the PDCP entity is required.